Every voice has a natural range --- the range of notes a person can create comfortably. Some people have larger vocal ranges than others and women's vocal ranges are typically higher than men's. If you're a singer and want to expand your range to hit either higher or lower notes, you may be able to do so. Results will vary from person to person and the amount of range you'll gain will differ as well.
Determine your natural range. If you've got a voice coach she'll be able to help you determine your natural range. For singers without a coach or trainer, there are several websites that can help you determine your vocal range. This will help you determine where to start.
Use vocal exercises to strengthen your current lower range. Sing scales and intervals at the lower end of your range that you're comfortable singing and gradually work lower with each exercise. For example, if you're doing intervals and you're singing a G then a C, go down to F sharp and B. Practice each interval or scale several times each day to strengthen your existing lower range.
Find your chest voice. If you're not familiar with the term "chest voice," keep lowering your scales until you feel your notes vibrating in your chest. This is your chest voice and it's where your lower notes will come from. Once you've figured out how to use your chest voice, practice scales and intervals and go lower and lower each day with your exercises.
Breathe deeply and use your diaphragm when singing. Although lower notes require less air than higher notes, you want to make sure your diaphragm is strong and engaged whenever you're singing to get the most power and best sound. When you breathe, make sure your chest and belly move out each time you inhale. This will allow you to take in the most air with each breath.
Figure out which note or notes correspond to your normal speaking voice. If you can speak it, you can sing it, so sit down at the piano and begin speaking. Find the note on the keyboard that corresponds to the tone you use most when speaking. If this note is lower than your current range, continue to work on vocal exercises to open up this range. Make sure you stand up straight in a comfortable pose and relax when practicing. Tension and bad posture will prevent you from reaching your goal.
Practice regularly. Training your voice to reach new notes takes time and practice, but if you work at it regularly, your hard work will pay off and you will extend your lower vocal range.